We are using Hibernate Spring MVC with OpenSessionInView filter.
Here is a problem we are running into (pseudo code)
transaction 1
load object foo
transaction 1 end
update foo's properties (not calling session.save or session.update but only foo's setters)
validate foo (using hibernate validator)
if validation fails ?
go back to edit screen
transaction 2 (read only)
load form backing objects from db
transaction 2 end
go to view
else
transaction 3
session.update(foo)
transaction 3 end
the problem we have is if the validation fails
foo is marked "dirty" in the hibernate session (since we use OpenSessionInView we only have one session throughout the http request), when we load the form backing objects (like a list of some entities using an HQL query), hibernate before performing the query checks if there are dirty objects in the session, it sees that foo is and flushes it, when transaction 2 is committed the updates are written to the database.
The problem is that even though it is a read only transaction and even though foo wasn't updated in transaction 2 hibernate doesn't have knowledge of which object was updated in which transaction and doesn't flush only objects from that transaction.
Any suggestions? did somebody ran into similar problem before
Update: this post sheds some more light on the problem: http://brian.pontarelli.com/2007/04/03/hibernate-pitfalls-part-2/
You can run a get on foo to put it into the hibernate session, and then replace it with the object you created elsewhere. But for this to work, you have to know all the ids for your objects so that the ids will look correct to Hibernate.
There are a couple of options here. First is that you don't actually need transaction 2 since the session is open you could just load the backing objects from the db, thus avoiding the dirty check on the session. The other option is to evict foo from the session after it is retrieved and later use session.merge() to reattach it when you what your changes to be stored.
With hibernate it is important to understand what exactly is going on under the covers. At every commit boundary it will attempt to flush all changes to objects in the current session regardless of whether or not the changes where made in the current transaction or any transaction at all for that matter. This is way you don't actually need to call session.update() for any object that is already in the session.
Hope this helps
There is a design issue here. Do you think an ORM is a transparent abstraction of your datastore, or do you think it's a set of data manipulation libraries? I would say that Hibernate is the former. Its whole reason for existing is to remove the distinction between your in-memory object state and your database state. It does provide low-level mechanisms to allow you to pry the two apart and deal with them separately, but by doing so you're removing a lot of Hibernate's value.
So very simply - Hibernate = your database. If you don't want something persisted, don't change your persistent objects.
Validate your data before you update your domain objects. By all means validate domain objects as well, but that's a last line of defense. If you do get a validation error on a persistent object, don't swallow the exception. Unless you prevent it, Hibernate will do the right thing, which is to close the session there and then.
What about using Session.clear() and/or Session.evict()?
What about setting singleSession=false on the filter? That might put your operations into separate sessions so you don't have to deal with the 1st level cache issues. Otherwise you will probably want to detach/attach your objects manually as the user above suggests. You could also change the FlushMode on your Session if you don't want things being flushed automatically (FlushMode.MANUAL).
Implement a service layer, take a look at spring's #Transactional annotation, and mark your methods as #Transactional(readOnly=true) where applicable.
Your flush mode is probably set to auto, which means you don't really have control of when a DB commit happens.
You could also set your flush mode to manual, and your services/repos will only try to synchronize the db with your app when you tell them to.
Related
We have a somewhat huge application which started a decade ago and is still under active development. So some parts are still in J2EE 1.4 architecture, others using Java EE 5/6.
While testing some new code, I realized that I had data inconsistency between information coming in through old and new code parts, where the old one uses the Hibernate session directly and the new one an injected EntityManager. This led to the problem, that one part couldn't see new data from the other part and thus also created a database record, resulting in primary key constraint violation.
It is planned to migrate the old code completely to get rid of J2EE, but in the meantime - what can I do to coordinate database access between the two parts? And shouldn't at some point within the application server both ways come together in the Hibernate layer, regardless if accessed via JPA or directly?
You can mix both Hibernate Session and Entity Manager in the same application without any problem. The EntityManagerImpl simply delegates calls the a private SessionImpl instance.
What you describe is a Transaction configuration anomaly. Every database transaction runs in isolation (unless you use REAN_UNCOMMITED which I guess it's not the case), but once you commit it the changes are available from any other transaction or connection. So once a transaction is committed you should see al changes in any other Hibernate Session, JDBC connection or even your database UI manager tool.
You said that there was a primary key conflict. This can't happen if you use Hibernate identity or sequence generator. For the old hi-lo generator you can have problems if an external connection tries to insert records in the same table Hibernate uses an old hi/lo identifier generator.
This problem can also occur if there is a master/master replication anomaly. If you have multiple nodes and there is no strict consistency replication you can end up with primar key constraint violations.
Update
Solution 1:
When coordinating the new and the old code trying to insert the same entity, you could have a slect-than-insert logic running in a SERIALIZABLE transaction. The SERIALIZABLE transaction acquires the appropriate locks on tour behalf and so you can still have a default READ_COMMITTED isolation level, while only the problematic Service methods are marked as SERIALIZABLE.
So both the old code and the new code have this logic running a select for checking if there is already a row satisfying the select constraint, only to insert it if nothing is found. The SERIALIZABLE isolation level prevents phantom reads so I think it should prevent constraint violations.
Solution 2:
If you are open to delegate this task to JDBC, you might also investigate the MERGE SQL statement, if your current database supports it. Basically, this is an upsert operation issuing an update or an insert behind the scenes. This command is much more attractive since you can still run it with even on READ_COMMITTED. The only drawback is that you can't use Hibernate for it, and only some databases support it.
If you instanciate separately a SessionFactory for the old code and an EntityManagerFactory for new code, that can lead to different value in first level cache. If during a single Http request, you change a value in old code, but do not immediately commit, the value will be changed in session cache, but it will not be available for new code until it is commited. Independentely of any transaction or database locking that would protect persistent values, that mix of two different Hibernate session can give weird things for in memory values.
I admit that the injected EntityManager still uses Hibernate. IMHO the most robust solution is to get the EntityManagerFactory for the PersistenceUnit and cast it to an Hibernate EntityManagerFactoryImpl. Then you can directly access the the underlying SessionFactory :
SessionFactory sessionFactory = entityManagerFactory.getSessionFactory();
You can then safely use this SessionFactory in your old code, because now it is unique in your application and shared between old and new code.
You still have to deal with the problem of session creation-close and transaction management. I suppose it is allready implemented in old code. Without knowing more, I think that you should port it to JPA, because I am pretty sure that if an EntityManager exists, sessionFactory.getCurrentSession() will give its underlying Session but I cannot affirm anything for the opposite.
I've run into a similar problem when I had a list of enumerated lookup values, where two pieces of code would check for the existence of a given value in the list, and if it didn't exist the code would create a new entry in the database. When both of them came across the same non-existent value, they'd both try to create a new one and one would have its transaction rolled back (throwing away a bunch of other work we'd done in the transaction).
Our solution was to create those lookup values in a separate transaction that committed immediately; if that transaction succeeded, then we knew we could use that object, and if it failed, then we knew we simply needed to perform a get to retrieve the one saved by another process. Once we had a lookup object that we knew was safe to use in our session, we could happily do the rest of the DB modifications without risking the transaction being rolled back.
It's hard to know from your description whether your data model would lend itself to a similar approach, where you'd at least commit the initial version of the entity right away, and then once you're sure you're working with a persistent object you could do the rest of the DB modifications that you knew you needed to do. But if you can find a way to make that work, it would avoid the need to share the Session between the different pieces of code (and would work even if the old and new code were running in separate JVMs).
This seems like it would come up often, but I've Googled to no avail.
Suppose you have a Hibernate entity User. You have one User in your DB with id 1.
You have two threads running, A and B. They do the following:
A gets user 1 and closes its Session
B gets user 1 and deletes it
A changes a field on user 1
A gets a new Session and merges user 1
All my testing indicates that the merge attempts to find user 1 in the DB (it can't, obviously), so it inserts a new user with id 2.
My expectation, on the other hand, would be that Hibernate would see that the user being merged was not new (because it has an ID). It would try to find the user in the DB, which would fail, so it would not attempt an insert or an update. Ideally it would throw some kind of concurrency exception.
Note that I am using optimistic locking through #Version, and that does not help matters.
So, questions:
Is my observed Hibernate behaviour the intended behaviour?
If so, is it the same behaviour when calling merge on a JPA EntityManager instead of a Hibernate Session?
If the answer to 2. is yes, why is nobody complaining about it?
Please see the text from hibernate documentation below.
Copy the state of the given object onto the persistent object with the same identifier. If there is no persistent instance currently associated with the session, it will be loaded. Return the persistent instance. If the given instance is unsaved, save a copy of and return it as a newly persistent instance.
It clearly stated that copy the state(data) of object in database. if object is not there then save a copy of that data. When we say save a copy hibernate always create a record with new identifier.
Hibernate merge function works something like as follows.
It checks the status(attached or detached to the session) of entity and found it detached.
Then it tries to load the entity with identifier but not found in database.
As entity is not found then it treat that entity as transient.
Transient entity always create a new database record with new identifier.
Locking is always applied to attached entities. If entity is detached then hibernate will always load it and version value gets updated.
Locking is used to control concurrency problems. It is not the concurrency issue.
I've been looking at JSR-220, from which Session#merge claims to get its semantics. The JSR is sadly ambiguous, I have found.
It does say:
Optimistic locking is a technique that is used to insure that updates
to the database data corresponding to the state of an entity are made
only when no intervening transaction has updated that data since the
entity state was read.
If you take "updates" to include general mutation of the database data, including deletes, and not just a SQL UPDATE, which I do, I think you can make an argument that the observed behaviour is not compliant with optimistic locking.
Many people agree, given the comments on my question and the subsequent discovery of this bug.
From a purely practical point of view, the behaviour, compliant or not, could lead to quite a few bugs, because it is contrary to many developers' expectations. There does not seem to be an easy fix for it. In fact, Spring Data JPA seems to ignore this issue completely by blindly using EM#merge. Maybe other JPA providers handle this differently, but with Hibernate this could cause issues.
I'm actually working around this by using Session#update currently. It's really ugly, and requires code to handle the case when you try to update an entity that is detached, and there's a managed copy of it already. But, it won't lead to spurious inserts either.
1.Is my observed Hibernate behaviour the intended behaviour?
The behavior is correct. You just trying to do operations that are not protected against concurrent data modification :) If you have to split the operation into two sessions. Just find the object for update again and check if it is still there, throw exception if not. If there is one then lock it by using em.(class, primary key, LockModeType); or using #Version or #Entity(optimisticLock=OptimisticLockType.ALL/DIRTY/VERSION) to protect the object till the end of the transaction.
2.If so, is it the same behaviour when calling merge on a JPA EntityManager instead of a Hibernate Session?
Probably: yes
3.If the answer to 2. is yes, why is nobody complaining about it?
Because if you protect your operations using pessimistic or optimistic locking the problem will disappear:)
The problem you are trying to solve is called: Non-repeatable read
I get an entity 'A' using
getHibernateTemplate().get(A.class, 100)
from the database. Lets say this entity 'A' has a property 'value' 200 in the database.
Now, in my Java code, I change a property for this entity. lets say, I change the 'value' property to '500' and then add it to some list.
Now, If I again do getHibernateTemplate().get(A.class, 100) for the same Entity, I am getting the updated entity(that has a value of 500). How do I force hibernate to get me the entity from the database, but not the one updated in my code?
Is this what is called as 'First Level Caching'?
Your assumption (about first level caching) is correct. As for example stated here: Interface Session:
The main runtime interface between a Java application and Hibernate.
This is the central API class abstracting the notion of a persistence service.
Or here Chapter 2. Architecture; 2.1. Overview
Extract: Session (org.hibernate.Session)
A single-threaded, short-lived object representing a conversation between the application and the persistent store. It wraps a JDBC
connection and is a factory for Transaction. Session holds a mandatory
first-level cache of persistent objects that are used when navigating
the object graph or looking up objects by identifier.
And also, you can see the methods available for us, to remove an object form the session:
evict(Object object):
Remove this instance from the session cache.
refresh(Object object):
Re-read the state of the given instance from the underlying database.
clear():
Completely clear the session.
And many more. Evict in this case should be working. We have to take the current instance ('A') and explicitly Evict it from the session.
If we've already loaded some/more stuff, and we do not know, what to Evict(), we simply need to get the fresh data. Then we can call Clear() to completely reset the session and start again.
This is a bit radical, because none of the objects in the session will be updated/inserted on session Flush()... but it could be what we want in this scenario (very often used for testing... load, clear... change and flush)
I suggest searching Google for hibernate commit, flush, and detach and reading up on when they write to the database. Better yet, I recommend reading a good book on Hibernate if you haven't already done so (search amazon.com for good reviews on a book) to get a good grasp of the technology.
My reason for responding to this post is not to answer your question directly, but suggest that you edit your hibernate.cfg.xml file and set the following to true:
< property name="hibernate.show_sql" > false < /property >.
This will cause a display to your console window to list when every sql statement that is sent to the database. This way, you can see exactly when a write to the database occurs. You can then experiment with what you research/read and verify it works as you expect.
I am playing with a standard optimistic concurrency control scenario with extended session / automatic versioning. I have an entity which I load in the first transaction, present to user for modification and save in the second one, both transactions sharing the same session. After the entity is somehow modified session.flush() at the end of the second transaction may throw a StaleObjectStateException in case a version inconsistency is detected meaning that a concurrent transaction has saved a next version of the entity in between.
I want to handle such an error in a most simple way -- just to reload entity losing current changes and continue with editing and saving again. First I tried this:
session.refresh(entity);
but after I modify and attempt to save this refreshed entity, I still get the same StaleObjectStateException, even though it does get refreshed and the version number appears consistent; yes I know that using refresh() in extended sessions is discouraged, but don't understand why. Is this behavior related to the reason it is discouraged?
Next I tried the following way to avoid using session.refresh():
session.evict(entity);
entity = session.load(MyEntity.class, id);
but it still results in StaleObjectStateException being raised at saving the entity which is not indeed stale.
The only way I managed to cope with the exception is this:
session.clear();
entity = session.load(MyEntity.class, id);
but isn't session.clear() the same as session.evict() pertaining to my concrete entity?
To resume, my questions are:
Why is StaleObjectStateException still thrown on a reloaded entity unless session.clear() is done?
What is the correct way to reload an entity which has already been loaded in the same session and why is refresh() bad? Is there something wrong with this approach to implement conversation?
I'm using Hibernate 4.1.7.Final, with no second-level cache.
My apologies if my question is repeating, but I fail to find a profound explanation...
When you get an exception in a session, than that session instance is broken. You cannot use that instance any more and you have to throw it away and create a new instance. The exception is not reset (as you can see you get the same exception again thought logically this should not happen). This is a general rule for using hibernate sessions. The reason for this is, hibernate does not always see why an exception appears and the state of the session instance may be inconsistent.
I don't know why it works after clear(). This may be accidentally. It is more prudent to use a new instance.
If you use a StatelessSession, then you don't have this restriction, but stateless sessions have other disadvantages, for example no caching.
My understanding of Hibernate is that as objects are loaded from the DB they are added to the Session. At various points, depending on your configuration, the session is flushed. At this point, modified objects are written to the database.
How does Hibernate decide which objects are 'dirty' and need to be written?
Do the proxies generated by Hibernate intercept assignments to fields, and add the object to a dirty list in the Session?
Or does Hibernate look at each object in the Session and compare it with the objects original state?
Or something completely different?
Hibernate does/can use bytecode generation (CGLIB) so that it knows a field is dirty as soon as you call the setter (or even assign to the field afaict).
This immediately marks that field/object as dirty, but doesn't reduce the number of objects that need to be dirty-checked during flush. All it does is impact the implementation of org.hibernate.engine.EntityEntry.requiresDirtyCheck(). It still does a field-by-field comparison to check for dirtiness.
I say the above based on a recent trawl through the source code (3.2.6GA), with whatever credibility that adds. Points of interest are:
SessionImpl.flush() triggers an onFlush() event.
SessionImpl.list() calls autoFlushIfRequired() which triggers an onAutoFlush() event. (on the tables-of-interest). That is, queries can invoke a flush. Interestingly, no flush occurs if there is no transaction.
Both those events eventually end up in AbstractFlushingEventListener.flushEverythingToExecutions(), which ends up (amongst other interesting locations) at flushEntities().
That loops over every entity in the session (source.getPersistenceContext().getEntityEntries()) calling DefaultFlushEntityEventListener.onFlushEntity().
You eventually end up at dirtyCheck(). That method does make some optimizations wrt to CGLIB dirty flags, but we've still ended up looping over every entity.
Hibernate takes a snapshot of the state of each object that gets loaded into the Session. On flush, each object in the Session is compared with its corresponding snapshot to determine which ones are dirty. SQL statements are issued as required, and the snapshots are updated to reflect the state of the (now clean) Session objects.
Take a look to org.hibernate.event.def.DefaultFlushEntityEventListener.dirtyCheck
Every element in the session goes to this method to determine if it is dirty or not by comparing with an untouched version (one from the cache or one from the database).
Hibernate default dirty checking mechanism will traverse current attached entities and match all properties against their initial loading-time values.
You can better visualize this process in the following diagram:
These answers are incomplete (at best -- I am not an expert here). If you have an hib man entity in your session, you do NOTHING to it, you can still get an update issued when you call save() on it. when? when another session updates that object between your load() and save(). here is my example of this: hibernate sets dirty flag (and issues update) even though client did not change value